The application of paint to steel strip in large scale, continuously operating, steel finishing mills is a highly developed art.
Typically, the substrate strip is progressed through a coating station wherein liquid paint, comprising pigments and other paint solids dissolved in a solvent or otherwise dispersed in a liquid carrier, is applied to the substrate by a dipping, spraying, roller coating or like process for applying a liquid film to the substrate, which film is subsequently allowed or caused to evaporate to leave a solid paint coat on the substrate.
It is also known to apply paint composition to a heated substrate wherein the paint is applied as a liquid melted from a solid body of substantially solvent free paint composition by contact of the body with, or near approach of the body to, the hot substrate. In this context the term "liquid" includes high viscosity liquids, that may approach soft, plastic solids in nature, as well as easily flowing liquids.
That last mentioned mode of depositing liquid material on a substrate is referred to as "melt deposition" and the deposited liquid is referred to as a "melt deposit" hereinafter.
Previously the deposition rate of melt deposits was determined by controlling the contact pressure between the solid paint body and the substrate, while maintaining constant all of the many other parameters affecting the deposition rate. Such a process is described in U.S. Pat. No. 3,630,802 to Dettling.
The difficulty of accurately controlling all of those parameters makes it difficult to obtain constant deposition rates of low value when using Dettling type pressure controlled melt deposition processes. This lead to their replacement, in painting operations, by the melt deposition technique described in our Australian patent No. 667716.
Briefly stated, that Australian patent discloses depositing a polymer based coating composition onto a side of a substrate metal strip moving at a constant speed, by heating the strip to a temperature above the glass transition temperature of the composition, and driving a solid block of the composition towards the strip at a predetermined block speed.
It is then only necessary to control the block speed to cause a melt deposit to be applied to the strip at a precisely controlled deposition rate, without the need to closely control other operating parameters, in that each of those other parameters need only lie within a broad range of working values.
As is also disclosed in that Australian patent, the melt deposit may then be spread over the surface of the strip by a pressure roll to emerge as a smooth, wet coating on the strip.
Irrespective of the mode of deposition, the prior art has been restricted to the production of mono-chrome product, wherein a uniform coating is applied to the whole of at least one side of the substrate strip.